The present invention relates to a driving method of droplet ejection head, and particularly to a driving method of droplet ejection head which is capable of suppressing bending tail of a droplet ejected from a nozzle, and improving landing accuracy of the droplet without decreasing drive frequency of the ejection head.
As an example of the droplet ejection head for ejecting liquid as a droplet from a nozzle, well known is an inkjet recording head that records an image by ejecting minute droplets onto a recording medium.
This type of inkjet head imposes pressure into a channel filled with ink, protrudes an ink pillar from the nozzle, after that, pulls back the tail edge of the protruded ink pillar by pulling in a meniscus, and separates the ink pillar from the meniscus. By this way, as shown in FIG. 13, the ink droplet 200 flies toward the recording medium 300 from the nozzle 400.
The ink droplet 200, just after ejected from the nozzle 400, has the main droplet 201 showing almost spherical shape and the tail portion 202 long extended from the end of the main droplet 201. However, after that, the tail portion separates to become second order droplets 203 so-called satellites. These main droplet 201 and second order droplets 203 fly toward the recording medium 300, and land to record an image. At this time, if the flying direction of the main droplet 201 and the flying direction of the second order droplets 203 are the same, they land at the same place to cause no bad effect on the image. However, if the flying direction of the second order droplets 203 differs from that of the main droplet 201, the second order droplets land around the main droplet 201 as shown in FIG. 13 to cause deterioration of the image.
The difference between the flying direction of the second order droplet 203 and the main droplet 201 is caused by generation of bending at the tail portion 202 of the ink droplet 200 just after the ejection from the nozzle 400, the bending direction being different from the originally expected direction shown by the arrow in FIG. 13.
The cause of the generation of bending at the tail portion of the ink droplet being ejected from the nozzle is known to be the irregularity of nozzle inner surface. For instance, as shown in FIG. 14, if inclination of the nozzle inner surface 401 is not uniform but differs at a certain portion, there arises an imbalance of the surface tension of meniscus M at the nozzle inner surface 401 to apply a biasing force that bends the tail portion 501 of the ink pillar 500 protruded from the nozzle 400, the biasing force being imposed to the direction perpendicular to the originally expected flying direction, thus the bending at the tail portion is generated just after the separation of the ink droplet from the meniscus. Therefore, accurateness in the shape of the nozzle inner surface affects greatly to a stable ink droplet ejection with suppressed tail bending of the ink droplet.
However, the accuracy level required to the nozzle inner shape is extremely high, and it is extremely difficult to manufacture the nozzle inner surface such that sectional shape of the nozzle inner surface is accurately circular and symmetrical about the center of the nozzle, therefore to satisfy the requirement is hardly possible. Further, if in usage a foreign material attaches to the nozzle inner surface, this being difficult to be removed, and this foreign material sometimes causes to generate the bending of the tail portion of the droplet. Therefore, it is required to stably eject a droplet without bending at the tail portion of the droplet by the other way than keeping accurate nozzle inner shape.
There has been studied to suppress the bending at the tail portion of the droplet. The Patent Document 1 discloses the technology where after protruding the ink pillar from the nozzle by imposing a first pulse, protruding the meniscus from the nozzle by imposing a second pulse before separation of the droplet in order to separate the droplet at the top of the convex shaped meniscus, and thereby prevents the bending of the tail portion.
(Patent Document 1)
Unexamined Japanese Patent Application Publication No. H2-215537
According to the technology described in the Patent Document 1, since the droplet is separated after the meniscus is protruded from the nozzle, the tail portion of the droplet is not affected by the nozzle inner surface shape. However, since other than the first pulse for protruding the ink pillar the second pulse for protruding the meniscus is further applied, the drive frequency is reduced, which leads to a problem of decreasing of recording rate. Further, since the next ejection cannot be started unless canceling the residual pressure wave, application of a new pulse to cancel the residual pressure wave is needed, and further reduce of the drive frequency is caused.